The selection of a heart valve structure for right ventricle outflow tract (RVOT) reconstruction may present a major challenge in the treatment of many congenital heart diseases including, without limitation, tetralogy of Fallot with pulmonary atresia, truncus arteriosus, transposition of great arteries with pulmonary stenosis, and congenital aortic stenosis/insufficiency.
Heart valve structures that may be used for RVOT reconstruction in pediatric patients may consist of homografts, which may not be readily available in many cases, and xenografts, which may be expensive (frequently around $4,000-$5,000). After the invention of the cryopreservation process in early 1980s, and especially with the increased availability of a wide range of sizes, the homograft has frequently become the heart surgeon's heart valve structure of choice for the RVOT reconstruction. However, longitudinal studies have demonstrated that homografts may also necessitate heart valve structure replacement due to stenosis and insufficiency. Such complications may be caused by shrinkage and calcification, and may be especially problematic for younger patients.
Recently, new xenograft designs have been evaluated for RVOT reconstruction including a glutaraldehyde-fixed porcine aortic valve and root, and a glutaraldehyde-fixed segment of a bovine jugular vein with venous valve. Although the anatomical shape of the porcine prosthesis may fit well to the RVOT, stenosis and calcification issues may still persist when the prosthesis is implanted in children. Similarly, recent reports on the bovine heart valve structures suggest a significant early fibrotic ring formation at the distal anastomosis. Additionally, dramatic dilation of and regurgitation through a heart valve structure may occur in the setting of pulmonary hypertension or distal anastomotic ring. The most successful heart valve structures for RVOT reconstruction, the homograft and the bovine jugular vein, both have shown re-operation rates of around 10-20% after about only two years. Re-operation and re-intervention rates, especially for the bovine xenograft, appear to increase significantly with increasing time and decreasing conduit diameter.
Both homografts and xenografts may suffer from calcification, which may result in stenosis and insufficiency, leading to the need for re-operation and replacement of the heart valve structure. Additionally, studies suggest that bioprosthetic heart valve structures available for RVOT reconstruction i.e. both allografts and xenografts, may be ineffective due to poor hemodynamic performance and long-term complications, especially in very young patients. Even after bioprosthetic valve replacement is performed, frequent surgeries for RVOT reconstruction may be required until the individual reaches adulthood. The additional surgeries may be required due to recurrent stenosis/insufficiency caused by calcification or degenerative processes, as well as the relative stenosis due to somatic growth.
Artificial heart valve structures may be considered as an alternative to both homografts and xenografts. However, artificial mechanical valves may not generally be available for RVOT reconstruction for pediatric patients. One factor that may affect availability of such heart valve structures may include the difficulty of designing a valve structure which can deal with the very low pressures (which may be less than 20 mmHg in many cases) found in the pediatric RVOT. Additional design challenges may also include small conduit diameter, a high degree of curvature along the conduit path, and the need for conduit flexibility as the patient grows. Intensive bioengineering studies may be required to produce effective designs customized for the pediatric/neonatal population. In use, mechanical valves may have higher longevity when implanted in the pulmonary position compared to implantation in the aortic position, but may require aggressive anticoagulant therapy due to a higher risk of thrombosis.
In addition to those conditions disclosed above for which RVOT is indicated, other disorders may also benefit from implanted artificial heart valve structures. Hypoplastic Left Heart Syndrome (HLHS) is a rare and complex congenital heart disorder which may be extremely difficult to treat successfully. HLHS may be characterized by a hypoplastic left ventricle that is unable to maintain systemic circulation, a hypoplastic aortic arch and ascending aorta that require reconstruction, and a patent ductus arteriosus that may maintain systemic circulation of the lower body. In order to treat HLHS, three separate procedures may be required: a Norwood operation, a bidirectional Glenn procedure, and a Fontan procedure.
The Norwood operation typically involves connecting the base of the pulmonary artery to the aortic arch in order to re-direct blood flow to the systemic tract. In order to continue to provide circulation to the pulmonary tract, a shunt or conduit may be placed following the Norwood operation to provide blood flow to the pulmonary artery. At present, there are two typical options for such a shunt: a Blalock-Taussig (BT) shunt that may connect the aorta to the base of the pulmonary artery, and a Sano shunt (RV-PA conduit) that may be placed between the right ventricle and the pulmonary artery.
The placement of the BT shunt may result in blood flow from the aorta to the pulmonary artery during both systolic and diastolic phases. This constant flow due to the BT shunt may cause low systemic diastolic pressure that can potentially lead to early mortality. The RV-PA conduit may avoid the issue of the pulmonary tract constantly leaching blood flow from the systemic tract by connecting the pulmonary artery directly to the right ventricle, rather than the aorta. In this manner the RV-PA shunt can maintain higher systemic diastolic pressure than the BT shunt. However present RV-PA shunts contain no valves, so backflow may occur into the right ventricle. As a result of the backflow, right ventricular enlargement may occur leading eventually to the need for partial or total heart replacement.
Shunts used for the treatment of HLHS can be very small, normally having a diameter of around 4 mm. This can make extremely difficult the design and manufacturing of any heart valve structure containing such a conduit. Past attempts at using a simple valved conduit have been unsuccessful, as the placement and geometry of the valve have resulted in the valve sticking to the conduit. Valve sticking may result in thrombus formation and flow impedance, which often results in early patient mortality.
Therefore, there appears to be a significant need for a heart valve structure, encompassing a conduit and a heart valve leaflet structure, with long durability for use with neonatal and pediatric patients.